1. Field of the Invention
The present invention relates to a method for the production of a multi-layer identity card of plastic.
2. Description of the Related Art
The invention relates to a method for the production of a Multi-layer identity card of plastic. Such identity cards are widely employed as check or bank cards or as an authorization means for access to mobile radio communication systems. They comprise at least one card core layer, which is printed on at least one side and a covering layer applied to the printed the side of the card core layer. These card layers are joined together in a laminating process.
The covering layer, which is usually made transparent, has an adhesive coating thereon adhering to the printed card core layer, and serves inter alia for protecting the print and also as a means bearing additional information, which for instance is applied in the thermotransfer printing method.
For the card layers—both the card core layers and also the covering layers—various different materials come into question: for instance polyvinyl chloride (PVC), polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), polyethylene terephthalate (PET, PETG and PETF). The selection of the materials and accordingly of the card structure is dependent on different factors. These factors are for instance: the desired mechanical properties of the identity card to be manufactured, its behavior at high and low temperatures, the question of printability, the question of laser inscription and furthermore economic and ecological determinants.
Dependent on which card structure is selected, different laminating parameters (temperature, pressure and time) are predicative. Same are dependent more especially on which material is utilized for the card core layers.
It is known to apply an adhesive coating (adhesive promoter, adhesive primer) on the card core layer prior to printing, which is to ensure improved adhesion of the printing ink, i. e. printing takes place on an adhesive coated card core layer and not directly on the card core layer. This procedure—printing on an adhesive coated card core layer—is more particularly necessary in the case of so-called digital printing on plastic foils. In the case of digital printing the printing image to be applied to the plastic foil to be printed is temporarily held, that is to say with the aid of selective electrostatic charges, on the printing roller. This digital printing method, which is for example employed in lasers printers, has now only been employed for a short time so far for printing plastic foils for the manufacture of identity cards. The printing inks and printing parameters applicable here are different to those in printing methods (offset litho and screen printing) employed so far in a classical manner, in the range in question, the necessity of printing on an adhesive coated card core layer being the reason.
In the case of lamination of a multi-layer identity card formed in this fashion there is the following problem in the case of relatively high lamination temperatures (over 124° C.): above a certain lamination temperature there is an undesired displacement (shift or drift) of the adhesive coatings including the printed layer sandwiched between them in relation to the covering layer and the card core layer. Adhesive coatings customarily employed form a sort of lubricating film between the card core layer and the covering layer. This phenomenon is to be attributed to the rheological properties of the materials utilized.